The present invention relates to a circuit for use in scaling a time interval. The present invention also relates to a time-to-digital converter (TDC) which includes such a circuit.
There are many instances when it is desirable to either expand or compress a time interval. For example, when measuring timing differences between pulses from elementary particle detectors in high energy physics experiments, the timing differences, which can be on the order of about 15 to 50 nanoseconds, are usually magnified.
In the past, the expansion of such a time interval has been achieved using a circuit having two linear current sources of unequal amplitudes, an electronic switch and a capacitor. The two linear current sources are used to charge and discharge the capacitor, the larger current source being used to charge the capacitor during the input time interval. The capacitor voltage is then compared to a threshold voltage to produce the output time interval, the output time interval being the overall time during which the capacitor voltage is held above threshold. The circuit is normally coupled to a counter having a digital output. The counter is enabled during the period the capacitor is discharging so as to provide a digital signal output corresponding to that time period.
One of the problems with the above described system is that it is relatively slow. The reason it is relatively slow is that it uses a counter to measure the discharge time of a capacitor.
Accordingly it is an object of this invention to provide a new and improved system for scaling a time interval and producing a digital output corresponding to the scaled time interval.
It is another object of this invention to provide an improved time scaling circuit.
It is still another object of this invention to provide a system as described above which does not include a counter.
It is yet still another object of this invention to provide a time scaling circuit which does not include constant current sources.